This script keeps outputting "Touched is not a valid member of tool"?

ball = script.Parent
damage = 10

r = game:service("RunService")

last_sound_time = r.Stepped:wait()

function onTouched(hit)

	local now = r.Stepped:wait()

	if (now - last_sound_time > .1) then
		ball.Boing:play()
		last_sound_time = now
	else
		return
	end

	local humanoid = hit.Parent:findFirstChild("Humanoid")
	local zombie = hit.Parent:findFirstChild("Zombie")
	if humanoid~=nil then
		tagHumanoid(humanoid)
		humanoid.Health = humanoid.Health - damage
		wait(2)
		untagHumanoid(humanoid)
		connection:disconnect()
	else
		damage = damage / 5
		if damage < 2 then
			connection:disconnect()
		end
	end
end

function tagHumanoid(humanoid)
	-- todo: make tag expire
	local tag = ball:findFirstChild("creator")
	if tag ~= nil then
		local new_tag = tag:clone()
		new_tag.Parent = humanoid
	end
end

function tagZombie(zombie)
	-- todo: make tag expire
	local tag = ball:findFirstChild("creator")
	if tag ~= nil then
		local new_tag = tag:clone()
		new_tag.Parent = zombie
	end
end


function untagHumanoid(humanoid)
	if humanoid ~= nil then
		local tag = humanoid:findFirstChild("creator")
		if tag ~= nil then
			tag.Parent = nil
		end
	end
end

connection = ball.Touched:connect(onTouched)


t, s = r.Stepped:wait()
d = t + 5.0 - s
while t < d do
	t = r.Stepped:wait()
end

ball.Parent = nil

Line of error: connection = ball.Touched:connect(onTouched)

Not exactly sure whats wrong with it so if you can tell me how to fix it and explain why the error happens, that would be nice of you.

This means that your script is a child of a tool, and by setting ball to script.Parent you are reffering to that tool. You need to change that line or place your script inside a part. Here is an explaination of what you might be doing wrong